Below about 200 Hz, the lower the frequency of sound, the more it is perceived not only by vibration of the ear drum but also by touch receptors in the skin. This sensation is familiar to anyone who has “felt the beat” of strong dance music in the chest, or through the seat of a chair, or has simply rested a hand on a piano. The natural stimulus is both auditory and tactile, and a true reproduction of it is possible only when mechanical vibration of the skin accompanies the acoustic waves transmitted through the air to the ear drum.
The prior art in audio-frequency tactile transducers primarily employ axial shakers. FIG. 1 shows an exploded view of a prior art headphone set 10 that includes axial shaker 100, including moving mass 114 suspended on spiral-cut spring 112, stator 116, and voice coil 118. The construction of such axial shakers mimics conventional audio drivers in which the light paper cone is replaced with a heavier mass, and a more robust suspension is provided, typically a spiral-cut metal spring.
A drawback of this construction is the production of unwanted acoustic noise. This occurs because the axial shaker is mounted in the headphone ear cup with the motion axis pointed at the opening of the ear canal. FIG. 2A shows a perspective view of prior art headphone set 20 that includes an axial shaker that vibrates along the z-axis and stimulates the skin by plunging the ear cup against the side of the user's head. Axial movement of the mass causes a countermovement of the entire ear cup itself, which is typically sealed over the pinna. Thus, the same force that stimulates the skin under the ear cup cushions unfortunately also plunges air into the listener's ear canal, overwhelming the output of the audio driver and generating the excess unwanted acoustic noise.
FIG. 2B shows a graph illustrating the excess apparent bass audio generated by the prior art headphones of FIG. 2A. In particular, FIG. 2B illustrates that the relatively flat acoustic frequency response of the headphones alone (traces labeled “off”) is degraded when the inertial shaker is turned on to progressively stronger levels (traces labeled “on”). In this example, significant audio is added to the acoustic frequency response, causing an undesirable bump of 10-20 dB in the 50-100 Hz range. The result is a bass-heavy sound in which upper frequencies are underrepresented, and the user's perception is one of muffled, muddy sound.
The problem of uneven frequency response is typically made worse by a lack of mechanical damping. Leaving the system underdamped means that steady state signals near mechanical resonance achieve high amplitude, leading to a peaked response, and that the system rings after excitation is stopped, further degrading audio fidelity. Such a bump is evident in the frequency response of the prior art (FIG. 2B), where actuating the tactile transducer increases the acoustic output of the headphone 10 to 20 dB above the 90 dB Sound Pressure level that is indicated by the “0” reference line.
Another approach in the prior art, also problematic, is the use of un-damped eccentric rotating motors (“ERMs”) and un-damped linear resonant actuators (“LRAs”). Small, un-damped ERMs are incompatible with high-fidelity audio for a few reasons. First, it generally takes about 20 milliseconds to “spin up” an ERM to a frequency that produces an acceleration large enough to be felt. By then an impulse signal (for example, the attack of a kick drum) will have passed. Second, in an ERM the acceleration, which can be likened to a “tactile volume,” and frequency, which can be likened to a “tactile pitch” are linked and cannot be varied independently. This linkage is fundamentally incompatible with acoustic fidelity.
The main drawback of LRAs is the dependence on the “resonance,” that the name suggests. The devices are designed for tactile alerts, not fidelity, and so they resonate at a single frequency and produce perceptible vibration at only that frequency. For example a typical LRA might produce up to 1.5 g of acceleration at 175±10 Hz, but less than 0.05 g outside this 20 Hz range. Such a high Q-factor renders this sort of device useless for high fidelity reproduction of low frequency tactile effects in the 15-120 Hz range. Despite these problems, LRAs have been contemplated for vertical mounting in the top cushion of a headphone bow.
In addition to the limited frequency range of LRAs there is a another problem with using LRAs as audio-frequency tactile transducers is that a transducer mounted vertically between the headphone bow and the top of the skull flexes the bow. At a fine scale, this flexion makes the bow flap like the wings of a bird, where an ear cup is situated at each wing tip. The inward-outward component of the flapping plunges the ear cups against the sides of the wearer's head, again producing undesirable audio that competes with and distorts the acoustic response of the audio drivers in the ear cups.
To avoid such unwanted audio, one approach is to construct a low-profile, vibrating module which moves a mass in-plane (i.e. in the x-y plane of FIG. 2A). This approach minimizes the surface area that is oriented to cause the problematic axially directed acoustic radiation. When mounted in an ear cup, such an in-plane vibrating module produces motion parallel with the surface of the side of the head. This movement effectively shears the skin, creating tactile sensation with little effect on the volume of air trapped between the ear cup and the ear drum. Acoustic noise is therefore minimized. Consider the difference between sliding a glass over a table top (planar motion of the present invention) and plunging a toilet (axial motion, as used in prior art). Although this in-plane approach has been contemplated, the dielectric elastomer actuators proposed for this purpose are expensive and complex devices that require high voltage electronics. Another drawback of this approach was that no provision was made for critically damping those transducers. Accordingly the tactile acceleration frequency response was underdamped, with a claimed Q-factor of 1.5 to 3.
In terms of electromagnetic actuation, a relatively thin, flat arrangement of a coil and two magnets that produces planar motion has been disclosed. In particular, the vibration module includes a single-phased electromagnetic actuator with a movable member comprised of two parallel thin magnets magnetized transversely in opposite directions and connected by a magnet bracket, and a means for guiding the magnet bracket.
Although this general approach to providing electromagnetic actuation has not been applied in headphones, it has been applied to the problem of providing haptic feedback in computer input devices like joysticks. One such device includes an actuator comprising a core member having a central projection, a coil wrapped around the central projection, a magnet positioned to provide a gap between the core member and the magnet, and a flexible member attached to the core member and the magnet. In this design, the motion is guided by a parallel pair of flexures.
A drawback of this guiding approach is the vulnerability of flexures to buckling when loaded by longitudinal compression. Compressive longitudinal loads on the flexures arise naturally from the attraction of the magnet pair riding the flexures to iron flux guides on the coil side, such as the E-core that provides the central projection supporting the coil. Accordingly, the flexures must be thick enough to resist this load without Euler buckling. This thickness comes at the expense of increased stiffness in the motion direction, which may undesirably impede movement.
Despite this drawback, the general approach has been applied elsewhere. For example, a flexure-guided surface carrying the magnets has been contemplated for use as the face of a massaging element. One approach to mitigating the buckling problem is to bear the compressive load on an elastic element such as foam. Supporting the load with an elastic element has some undesirable drawbacks, however. The foam adds stiffness in the direction of travel, and may significantly increase the thickness of the assembly, since the foam layer must be thick enough that the maximum shear strain (typically <100%) allows adequate travel.
An alternative approach to suspending a moving element arranges the long axis of the flexures in the plane of a substantially flat transducer. Because slender flexures resist transverse shear loads more effectively than longitudinal compressive loads, thinner flexures may be used, providing less impediment to motion.
Therefore, there exists a need for novel audio-frequency tactile transducers and devices.